One of the most exciting technical developments in biology in recent years is the emergence of photochemical methods for controlling electrical activity with light. Our goal in this project is to develop a broadly applicable method for controlling other aspects of cellular function by generating tools for conferring light sensitivity on a broad range of cell surface receptors and ion channels. To meet this challenge we will use a modular approach, utilizing a single Universal Photoswitch as the key light-sensing component. The photoswitch contains at its core the small isomerizable azobenzene moiety, which shortens and lengthens in response to 380 and 500 nm light, respectively. We will use this photoswitch to indirectly regulate receptor and channel activity, through an adapter peptide, which contains a capture domain, which recognizes the short, but not the long configuration of the photoswitch, and a ligand domain, which contains a peptide activator or inhibitor of the targeted cell surface receptor or ion channel. The capture domain is kept constant among all adapter peptides, allowing control by a single Universal Photoswitch, but the nature of ligand domain is tailored to regulate a specific receptor. Several strategies will be used to translate light-dependent capture of the adapter peptide into receptor activation or inhibition. These include dimerizing the adapter peptide with a dimeric photoswitch to activate growth factor receptors, and delivering the adapter peptide, including the ligand, to a G-protein coupled receptor via an antibody-tethered photoswitch. Other strategies may be developed to activate different types of receptors and ion channels. We will test the effectiveness of the Universal Photoswitch approach on two example receptors: the TrkB receptor for brain-derived growth factor (BDNF), a member of the neurotrophin family of receptor tyrosine kinases, and the receptor for neuropeptide Y, which is a GPCR-type receptor. Generating a method for light-sensitive regulation of these receptors will allow examination of their roles in development and neural function in intact tissue with unprecedented precision, but more importantly, it will demonstrate the emergence of a powerful new technique for receptor regulation that can be applied to any cell surface protein for which a known peptide ligand exists.